The present invention relates generally to the formation of contacts in semi-conductor devices and in particular the invention provides a method of forming contacts to underlying regions in thin film devices.
Typically in traditional bulk material, photovoltaic devices contacts are made to either side of the device which is typically a single junction device.
When thin film devices are made they are formed on a substrate or superstrate which is typically an electrically non-conducting material such as glass making it necessary to connect to all layers of the device through one surface of the device. If the device is illuminated through the substrate this arrangement has the advantage of substantially removing the problem of contact shading, however forming contacts to thin underlying regions presents significant challenges which tend to require expensive solutions. The aim of using thin films in photovoltaic device fabrication is to achieve low cost devices, but this aim cannot be met if the fabrication process requires expensive high accuracy alignment steps to allow contact to underlying regions without producing junction shorts.
The present invention provides a method for forming a connection region for making electrical connection to a first doped semi-conductor region of a thin film semi-conductor device located beneath at least a second doped semi-conductor region of opposite dopant type, the method including the steps of:
forming the first semi-conductor region with a predominant first dopant type;
forming at least the second semi-conductor region over the first semi-conductor region with a predominant second dopant type of opposite polarity to the first dopant type, where in the connection region, the number of excess dopants of the predominant dopant type per unit area of the film in the second semi-conductor region is lower than the number of excess dopants of the predominant dopant type per unit area of the film in the corresponding first semi-conductor region;
heating a column of semi-conductor material passing through the second semi-conductor region and into the first semi-conductor region in an area of the connection region of the semi-conductor device where a contact to the first semi-conductor region is required, the heating step causing the material in the first and second semi-conductor regions to increase in temperature whereby dopant mobility is increased.
Preferably, the heating step causes the material in the first and second semi-conductor region to melt, whereby dopant atoms of the first dopant type migrate from the first semi-conductor region to the second semi-conductor region, the dopant concentration of the first semi-conductor region before melting, being sufficiently higher than the dopant concentration of the second semi-conductor region that after redistribution of the dopant atoms in the column during the melting step, the predominant dopant type in all semi-conductor regions of the columns is the same as that of the first semi-conductor region.
In the preferred embodiment, the melting step is performed using a laser and may be performed either without significant ablation or with minimum ablation of the surface or alternately, an ablating step may be performed concurrently to open the column. The laser may be controlled to provide a variable output with time, either in a single pulse or a sequence of pulses to control the rate of melting and ablation.
The doping concentration of the heavily doped first dopant type region must be high enough to enable the dopant type in the other regions to be dominated by the dopant in the heavily doped region after migration has occurred. This requires, in the connection area, the total dopants in the first dopant type region to be greater than the total dopants in the other regions. Typically, the ratio of total dopants is greater than 2:1. In typical embodiments, the first dopant type will be present in the first semi-conductor region in the range of 2xc3x971014 to 1xc3x971016 atoms/cm2 and the second dopant type will be present in the second semi-conductor region in the range of 1xc3x971014 to 5xc3x971015 atoms/cm2.
In typical embodiments, the combined thickness of the semi-conductor layers is in the range 0.1 to 10 xcexcm and preferably in the range 0.5 to 5 xcexcm.
In one embodiment of the invention, after formation of the column of first dopant type material, a further laser step is used to form an opening into the column, leaving an outside wall of the column intact to isolate the second semi-conductor region from the opening, and metal is then formed in the opening to provide contact to the underlying first semi-conductor region via the remaining wall of the column. In the alternative embodiment where the opening is formed as part of the melting step, the metal is formed after the combined melting/ablating step.
In a second embodiment of the invention, metal is formed over the column to make contact with the column and with the underlying first semi-conductor region via the column.
In a further use of the invention, metallisation is not performed and the doped columns are used to isolate buried layer contact areas from other areas of the cell to minimise the possibility of cell performance degradation or failure due to shorting of the junction adjacent to the contact.
According to a second aspect, the present invention provides a method of forming an electrical connection to a first doped semi-conductor region of a thin film semi-conductor device located beneath at least a second doped semi-conductor region of opposite dopant type, the method including the steps of:
forming the first semi-conductor region having a first dopant type;
forming at least the second semi-conductor region over the first semi-conductor region, the second semi-conductor region having a second dopant type of opposite dopant polarity to the first dopant type;
forming a metal layer over at least an area of the semi-conductor material in a contact region where a contact is required to the first semi-conductor region;
heating the metal layer in the contact region whereby the metal and the underlying semi-conductor material melt at least down to the first semi-conductor region such that at least a part of the metal layer alloys with the underlying material to form a column of metal alloy between the first semi-conductor region and the top surface of the device.
In one method according to the second aspect of the invention, the heating step is performed with a laser to locally heat a small region of the metal layer. In another method according to the second aspect of the invention, the metal layer is restricted to the contact region and the heating step comprises a thermal annealing step whereby the entire top surface is heated, the localised metallisation in the contact region being caused to melt and alloy with the underlying regions during the annealing step.
Preferably, the metal layer in the contact region is only partly consumed in the alloying step whereby the remaining metal is available to carry current into or out of the cell. Alternatively if all of the metal is consumed in the alloying step, further metal may be formed in a subsequent step.
In preferred embodiments, there will be a dielectric layer (such as silicon oxide or silicon nitride) formed over the active regions in which case the metal layer is formed over the dielectric layer and the alloying step will produce an alloy of the metal with dielectric material in the dielectric layer as well as an alloy or mixed composite of the metal with the semi-conductor material in the semi-conductor material.